The present invention relates to a device for underwater exploration or geophysical reconnaissance intended in particular but not exclusively for yo-yo seismic shooting.
Yo-yo seismic shooting is an operation during which the tool on the sea bed, on a sled, needs to be kept stationary for the length of time required to perform its operations while a surface vessel continues to advance at constant speed, with the tool or the sled being connected to the vessel by a cable. It is thus a question of transforming the continuous motion of the vessel into motion of the vehicle towed along the sea bed that is discontinuous, although cyclical. It thus possible to obtain the information required at some determined pitch. This operation is commonplace and it requires the tool-and-sled vehicle to remain stationary for a certain length of time, and then, on being raised, to be moved and again held stationary. That is to say it proceeds in jumps.
The solution presently in use consists in winding the cable on a special xe2x80x9cyo-yoxe2x80x9d winch. The winch ensures that the towed vehicle moves in discontinuous manner, and it suffers from major drawbacks associated with the technical specifications associated with yo-yo type operations: paying out speed of several meters per second, accurate monitoring of the length of cable paid out, instantaneous traction forces, accurate servo-control to maintain the rate, etc. The winches usually installed on surface vessels do not comply with these specifications. Special winches therefore need to be manufactured and secured on vessels, thereby giving rise to constraints that are expensive in terms of cost and in terms of on-site operations. Furthermore, such winches are heavy and bulky.
A first object of the invention is to mitigate that drawback and to enable yo-yo type operations to be performed without recourse to using a specialized winch.
U.S. Pat. No. 3,549,129 describes apparatus mounted on the deck of a ship and designed to compensate the effects of relative movement between a surface vessel subjected to swell and an operational device that must be kept at constant level. That apparatus includes a pantograph and operates continuously, but compensates movements of small amplitude only.
According to the invention, the yo-yo apparatus comprises a link cable between a surface vessel (N) advancing at substantially constant speed and a sea bed sled (T), the cable being wound on a winch mounted on the deck of the vessel, wherein a block-and-tackle unit is located on the deck between the winch and the vehicle, the block-and-tackle unit being fixed to a frame for letting out and taking in the length of cable required to enable the sled to move in jumps.
According to another characteristic of the invention, the block-and-tackle unit is constituted by two sets of sheaves over which the cable passes, a first set of sheaves being fixed, and the other set being capable of being moved away from or towards the first set under drive from an actuator fixed on the block-and-tackle unit frame.
The block-and-tackle unit actuated by an actuator enables the cable recovery and release operations to be performed quickly at the desired moments.